Metal coating



Patented Jan. 29, 1952 METAL cos'rme Hiram S. Lukens, Philadelphia, Pa., assignor to the United States of America as represented by the Secretary of War No Drawing.

Application July 1, 1947,

Serial No. 758,467

8 Claims.

This invention relates to metal coating, and more particularly to the application of an extremely thin metal coating to a base which completely seals the base against the outside.

It is well known in the art to protect a base from the outside by applying thereto a thin coat of a non-oxidizable or slowly oxidizable metal. An outstanding example of this is the application of a silver coat to an iron or steel base. By electrodeposition, thin non-porous coats of a thickness of the order of .0015 inch may be formed. However, if the coat is still thinner, it fails to protect the base because of the existence of pores in a coat below such thickness.

I have discovered a method for sealing the pores of an extremely thin metal coat, which makes possible the employment of a coat having only one-third to one-half the thickness of what has heretofore been considered the minimum thickness for complete protection of the base. This method essentially consists in applying an extremely thin, though porous, metal coat to the base, and polarizing the coated object in a suitable electrolyte solution to form on the surface of a porous metal coating a film of a compound of such properties that it can be subsequently fused, thereby sealing the pores in the porous metal coating.

Consequently, one of the principal objects of my invention is the formation of a metal coat of extreme thinness, whose pores are sealed by a compound of the coating metal.

A further object of my invention is the electrochemical formation of a fusible salt on the surface of an extremely thin metal coat, with subsequent fusion to seal the pores of the coat.

Another object of my invention is a saving in the cost of noble metals used for coating a base material.

A further object of my invention is the provision of an extremely thin non-porous coat on a base, which coat consists of a metal and of a water-insoluble salt of such metal, the color of the salt resembling the color of the metal so as to give the appearance of a homogeneous film.

Further objects and advantages of my invention will appear as the description of my process, the product produced thereby, and of several preferred examples proceeds.

In accordance with my invention, the base is first coated with a thin coat of the coating metal, preferably but not necessarily by electrodeposition. The coat is intentionally of such thinness that it is not free from pores. Thus, in the case of an electrodeposited silver coat, the coatmay be kept below a thickness of .0015 inch, which latter thickness is commonly considered to be the lower limit for a nonporous electrodeposited silver coat. I then immerse the coated article in an electrolytic bath and connect it as the anode. The electrolytic bath is a solution of an ionizable compound whose negative ions or radicals are capable of combining with the coating metal at the anode to form a water-insoluble fusible salt. Thus, in the case of a silver coated article, a suitable electrolytic bath is a Water solution of hydrochloric acid or of sodium chloridewhich, upon'passage of the current through the system, results in the formation of silver chloride on the silver coat. By selecting a solution of another halide such as sodium bromide or sodium iodide, the corresponding halogen salt may be formed by combination with the silver coat at the anode; inasmuch as silver chloride, silver iodide, and silver bromide are water-insoluble and fusible, they are suitable for the purpose of my invention. On the other hand, silver fluoride is water-soluble, and consequently a fluoride solution is unsuitable for use as an electrolytic bath for the purpose of my invention.

After a brief passage of the current through the system, the coated article, which forms the anode, is withdrawn after part of the metal has'been converted into the corresponding salt. The coated article is then subjected to a temperature above the melting point of the salt where the melting point of the salt is below that of the metal, or to a temperature above the melting point of the metal where the melting point of the salt is above the melting point of the metal. In the former case, i. e., where the melting point of the salt is below the melting point of the metal, the elevated temperature is kept below the melting point of the metal itself; this exposure causes the salt to melt and to flow into the pores of the thin metal coat. In the latter case, i. e., Where the melting point of the salt is above that of the metal, the metal itself when heated above its melting point will evenly fill out the interstices between the electrochemically formed salt deposit.

After cooling, a non-porous coat is found to have formed on the base. It may be buffed or burnished to give it a metallic appearance.

Among numerous uses for my invention may be mentioned, for instance, the electroplating of silverware.

The following specific examples illustrate various ways in which my invention may be practiced, but are not to be deemed to constitute an exclusive enumeration of the materials .or

3 methods suitable for the purposes of my invention.

Example I A porous silver coat of a thickness of the order of .0005. to .00075 inch is electrodeposited upon steel. The coated object is immersed into an electrolytic bath consisting of a water solution of 50 c. c. of concentrated hydrochloric acid in 450 c. c. of water (or in a sodium chloride solu-" The coated object is polarized-- as tion in water). the anode, and a current of .4 amp/sq. dcm. is permitted to flow through the system for 10minutes at room temperature. The coated object is then removed, and a very thin film of silver chloride will be found to have formed? on the silver. coat. The coated object is washed, dried, and heated to a temperature of 455 C. (the. melting. point of silver chloride) or somewhat above, but below .the melting point of silver (960.5 C.). The silver chloride becomes limpid and flows into thepores of. the silver coat. The object is then removed from the source of heat and permitted to. cool. Upon cooling, a non-porous very thin film of silver and silver chloride of a metallic or white color, depending on the thickness of the fused silver chloride, has been formed upon the base. Any excess silver chloride is removed by burniishing or'bufling, which gives the entire article a pleasing, silver-like-metallic appearance.

Example II Example HI ExampleII is carried out'by using asolution of sodium iodide as the electrolytic. bath. After removal from the bath, the. silverand silver iodide-covered. object is heated. to about 552 Ci, at which temperature. the silver iodide. fuses. The object is removed from the source of heat before any appreciable amount of the fused silver iodide has decomposed. Appearance and aftertreatment of the object are the same as in Example II.

Obviously; other metals and salts may be used tocarry out my invention: in accordance with the principle: set forth in this specification- It is necessary that the salt formed on the coat be waterinsoluble, to prevent. deterioration of the coat by leaching. The composition and concentration of the: electrolytic bath. may be varied to suit specific requirements of. eachindividual case. My invention is thus not restricted to any specific materials or quantities, or duration and density of thecurrent flow during the electrochemical treatment as set forth in. the foregoing examples, but encompasses. all such variations of my inventive thought as will. readily occur to the expert. I thus intend to claim my invention broadly, and tube. limited only by the appended claims.

I. claim:

L A process. of forminga thin non-porous coat .on a metallic. base, comprising depositing a porous 4 V inch on said base, immersing said coated base as an anode in an electrolytic solution capable of electrochemically forming with said silver a water-insoluble fusible silver salt having a lower melting point than said metallic base and silver, passing an electric current through said anode and through said electrolytic solution until said porous silver coat has been partly converted into a water-insoluble silver salt, and heating the surface of said coated base to at least the melting point. of said silver salt but below the melting point of said metallicv base and of silver, whereby said silver salt is melted and flows into the'pores of said thin porous silver coat and conjointly therewith forms a thin non-porous coat on said base.

2. A process of forming a thin non-porous coat on. ametallic base, comprisin depositing a porous silver coat of a thinness of about .0005 to about .00075 inch on said base, immersing said coated base as an, anode in an electrolytic solution capable of electrochemically forming with said silver a water-insoluble fusible silver salt having a lower melting point than said metallic base and silver; passing an electric current through said anode and through said electrolytic solution until said porous silver coat has been partly converted into a water-insoluble silver salt, and heating" the surface of said coated base to at least the melting point of said silver salt but below the melting point of said metallic base and of silver, whereby said silver salt is' melted and flows into the pores of said thin porous silver coat and conjointly therewith forms a thin non-porous coat on said base.

3. A process of forming a thin non-porous coat on a metallic base, comprising electrodepositing a thin porous silver coat on said base, immersing said. coated base as an anode in an electrolytic solution capable of electrochemically forming with said silver a water-insoluble fusible silver halide, passing an electric current through said anode and through said electrolytic solution until said porous silver coat has been partly converted into a water-insoluble silver halide, and heating the surface of said coated base to at least the melting point of said silver halide but below the melting point of silver and of said base, whereby said silver halide is melted and flows into the pores of said thin porous silver coat and conjointly therewith forms a thin non-porous coat on said base.

4. A process of forming a thin non-porous coat on a ferrous base, comprising depositing a porous silver coat of a thinness ofless than about .0015 inch, immersing said coated base as an anode in an electrolytic solution capable of electrochemically forming with said silver 2. water-insoluble fusible silver halide, passing an electric current through said anode and through said electrolytic solution until said porous silver coat has been partly converted into a water-insoluble silver halide, and heating the surface of said coated base to at least the melting point of said silver halide but below the melting point of silver, whereby said silver halide is melted and flows into the pores of said thin porous silver coat and conjointly forms a thin non-porous coat on said base.

5. A process according to claim 1, wherein said silver salt is silver chloride.

6. A process according to claim 3, wherein said silver halide is silver chloride.

7 A process according to claim 4, wherein said halide is a chloride.

8. A process according to claim 4, wherein said Number electrolytic solution is hydrochloric acid. 517,275 HIRAM SLLUKENS. 1,758,293 W 1,872,614 REFERENCES CITED 5 2,060,530 The following references are of record in the 2997300 file of this patent: 3

UNITED STATES PATENTS Number Name Date 1 26,978 Edwards Jan.31, 1860 3.684

Name Date Goodwin Mar. 27, 1894 Murray May 13, 1930 Wirshing et a1. Aug. 16, 1932 Reilly Nov. 10, 1936 Muller Oct. 26, 1937 Pierce Jan. 12, 1943 FOREIGN PATENTS Country Date Great Britain 1886 

1. A PROCESS OF FORMING A THIN NON-POROUS COAT ON A METALLIC BASE, COMPRISING DEPOSITING A POROUS SILVER COAT OF A THINNESS OF LESS THAN ABOUT .0015 INCH ON SAID BASE, IMMERSING SAID COATED BASE AS AN ANODE IN AN ELECTROLYTIC SOLUTION CAPABLE OF ELECTROCHEMICALLY FORMING WITH SAID SILVER A WATER-INSOLUBLE FUSIBLE SILVER SALT HAVING A LOWER MELTING POINT THAN SAID METALLIC BASE AND SILVER, PASSING AN ELECTRIC CURRENT THROUGH SAID ANODE AND THROUGH SAID ELECTROLYTIC SOLUTION UNTIL SAID POROUS SILVER COAT HAS BEEN PARTLY CONVERTED INTO A WATER-INSOLUBLE SILVER SALT, AND HEATING THE SURFACE OF SAID COATED BASE TO AT LEAST THE MELTING POINT OF SAID SILVER SALT BUT BELOW THE MELTING POINT OF SAID METALLIC BASE AND OF SILVER, WHEREBY SAID SILVER SALT IS MELTED AND FLOW INTO THE PORES OF SAID THIN POROUS SILVER COAT AND CONJOINTLY THEREWITH FORMS A THIN NON-POROUS COAT ON SAID BASE. 